Large pore volume olefin polymerization catalysts

ABSTRACT

Large pore volume (≧1.4 cc/g) silica is obtained by slowly drying an acidic (pH≦3.5) silica hydrogel. The gel is useful in connection with chromium as an ethylene polymerization catalyst.

The present invention relates to the polymerization of olefins. Morespecifically the present invention relates to a supported chromiumcatalyst comprising a large pore volume silica support. In accordancewith another aspect this invention relates to a process to produce sucha catalyst.

BACKGROUND OF THE INVENTION

One of the most important processes for the production of high densitypolyethylene is a catalytic polymerization process utilizing supportedchromium catalysts. Many variations and modifications of the basicprocess have been made over the years. There is a continuing need fordesigning specific catalysts on the basis of chromium which allow theproduction of polymers with predictable properties and which allow thecontrol of such properties.

THE INVENTION

One object of this invention is to provide a polymerization catalyst ofcontrolled pore volume.

Another object of this invention is to provide a large pore volumesilica for olefin polymerization catalysts enabling the control of themelt index and molecular weight distribution.

A further object of this invention is to provide a process for producinga silica xerogel or respectively an olefin polymerization catalystcomprising such a xerogel as the support, which xerogel has a high porevolume.

In accordance with this invention it has been found that by drying asilica-containing hydrogel having a low pH a large pore volume xerogelcan be produced.

In accordance with a first embodiment of this invention a process toproduce an olefin polymerization catalyst is therefore provided. Thisprocess comprises drying a silica-containing hydrogel having a pH of 3.5or less to obtain a xerogel with large pore volume. Prior to, during, orafter this drying step a chromium source is utilized to obtain achromium-containing gel and a chromium-containing xerogel is the productof the process.

A significant and preferred feature of this invention resides in thefact that the step of drying the low pH hydrogel is carried out veryslowly. This slow drying step appears to be of significant influence onthe stability of the silica walls that maintain the large pore sizepreventing the premature collapse of these pores. The drying ispreferably carried out slowly and relatively continuously over a periodof time of approximately 2 to 50 hours.

Under the preferred conditions for the slow drying the hydrogel-acidmixture is kept at a temperature ranging from about 90° to about 110° C.at atmospheric pressure until essentially all the water is evaporated.Thereafter the temperature can be increased to the calcinationtemperatures employed in activation if desired, e.g. 500° C. Dryingtimes at atmospheric pressure can range from about 2 to about 50 hours,preferably from 5 to 20 hours. If lower or higher pressures are employedduring the slow drying the temperature used will be similarly near theboiling point of water under the pressure chosen.

In accordance with another embodiment of this invention a polymerizationcatalyst of the supported chromium type is provided. In accordance withthis invention the silica xerogel support has a pore volume of aboveabout 1.4 cc/g as determined by the Innes method. The preferredcatalysts of this invention are those obtained by the process definedabove and in the following claims.

Yet another embodiment of this invention resides in a polymerizationprocess wherein one or more olefins are contacted under polymerizationconditions with a catalyst as defined. The preferred process usesethylene or ethylene together with one or more 1-olefins as the monomersfor the polymerization step.

The process involves contacting the purified silica hydrogel with acidof inorganic or organic nature sufficient to impart to the mixture a pHranging from about 0 to about 3.5, more preferably from about 0 to 3.The mixture is then slowly dried in an air oven, e.g. under a radiantheat source to obtain the large pore volume gels characteristic of theinstant invention.

The gels in combination with chromium compounds are subjected to (a)calcination of the mixtures at an elevated temperature in the presenceof molecular oxygen or (b) sequential calcination of the mixtures at anelevated temperature in a reducing ambient, e.g., carbon monoxide,followed by calcination in the presence of molecular oxygen. Therebycomposites are produced which are active catalysts for ethylenepolymerization. Calcination in an oxygen ambient is described in U.S.Pat. No. 2,825,721. Sequential calcination in a reducing ambient and anoxidizing ambient is disclosed in U.S. Pat. No. 4,151,122. Thesereferences are hereby incorporated into this application.

The silica hydrogels utilized in this invention are conventionallyproduced by contacting aqueous solutions of an alkali metal silicate,generally sodium silicate because of its ready availability andrelatively low cost, and an acid, generally a mineral acid such assulfuric acid although carbonic acid and organic acids such as aceticacid can also be employed. A minor amount of a water soluble compoundselected from an element of Groups IIIA, IVB, VB, VIB, VIIB, and VIII ofthe Periodic Table of the Elements as disclosed in the Handbook ofChemistry and Physics, Chemical Rubber Company 45th Edition (1964) p.B-2, and mixtures thereof can also be present. The amount of suchcompounds present, when employed, can range from any finite amount up toabout 20 weight percent (total) based on the dry composite with silicamaking up the balance. The resulting hydrogels are then aged, generallyat from about 100° to about 200° F. (38°-93° C.) for about 1 to about 24hours. The aged product is washed with water to reduce the alkali metalion content to about 0.1 weight percent or less. The purified hydrogenlis then admixed with the acid to attain the specified pH rangepreviously described and the mixture is slowly dried to obtain the gelsof this invention.

It has been found with coprecipitated silica-containing hydrogels suchas silica-titania cogel hydrogels which have larger pores and lessmechanical resistance to shrinkage on drying than silica hydrogels thatthe effect of adding an acid to increase pore volume is reduced relativeto the silica hydrogels. Thus, the maximum benefit achieved withincreasing pore volumes is observed with silica hydrogels containingfrom about 99 to 100 weight percent silica on a dry basis.

In one embodiment, a high pore volume silica gel containing a chromiumcompound, for example, can be produced by admixing a purified,acid-containing silica hydrogel in the specified pH range with a watersoluble chromium compound such as chromium(III)acetate,chromium(III)nitrate, chromium trioxide, and ammonium dichromate, andslowly drying the mixture. Alternately, the dry silica gel can beimpregnated with a chromium compound such aschromium(II)acetylacetonate, t-butyl chromate, dicumylchromium, etc.dissolved in a nonaqueous solvent such as n-hexane, pyridine, acetone,and the like. Nonaqueous impregnation is required in this method toavoid destroying the large pore volume material which can occur in thepresence of added water.

High pore volume silica gels in this invention are those essentially1000% silica gels which have pore volumes ranging from about 1.45 toabout 1.95 ml/g as determined with isopropanol according to the Innesmethod as described in Analytical Chemistry 28, 332-334 (1956). Prior toconducting the determinations the gels are calcined in air at about thesame temperature, e.g., 500° C. Control gels produced from the samehydrogel without the acid treatment, in contrast, have pore volumes ofabout 1.33 ml/g.

Inorganic acids employable in this invention are those which are watersoluble, sufficiently ionized to produce the pH level required in thehydrogels, and do not have a deleterious effect on the silicas or in theend use application. For the production of ethylene polymerizationcatalysts, specific but nonlimiting acids can be selected from amonghydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid,sulfamic acid, sulfuric acid, orthophosphoric acid and iodic acid.

Organic acids generally employable in this invention are those whichexhibit the same pH requirements as the inorganic acids. Specific butnonlimiting examples include acetic acid, formic acid, tartartic acid,citric acid, maleic acid, malic acid, malonic acid, succinic acid,gluconic acid, diglycolic acid, ascorbic acid, cyclopentanetetracarboxylic acid, and benzenesulfonic acid.

In general, those organic acids meeting the requirements of watersolubility, stability, acid strength, nondeleterious action as describedbefore also have pK values of about 4.76 or less as disclosed in Lange'sHandbook of Chemistry, 11th Edition (1973), Tables 5-7, 5-8. In otherwords, their acid strength is equal to or greater than that of aceticacid.

Acids such as sulfuric acid and hydrochloric acid are generallypreferred, however, because of their ready availability, relatively lowcost, great acid strength, and efficacy in the process.

The ethylene polymers which can be produced with the invention catalystsare normally solid homopolymers, copolymers terpolymers, etc. ofethylene containing up to about 20 mole % (total) of comonomers selectedfrom aliphatic 1-olefins having from 3 to about 10 carbon atoms and/or aconjugated diolefin having from 4 to about 12 carbon atoms. The polymerscan be converted by conventional means into useful articles such asfibers, films, sheets and containers.

Polymerization processes with the invention catalysts can be conductedbatchwise or continuously, and include gas phase, particle form andsoluble form modes of operation. For reasons of economy, a presentlypreferred polymerization process is the particle form process describedin U.S. Pat. No. 3,624,063. This patent, too, is incorporated herein byreference.

In such a polymerization process the olefin or mixture of olefins ispolymerized at a temperature within the range from about 150°-230 ° F.(65°-110° C.) and a pressure within the range from about 110 to about700 psia (0.75-4.8 MPa) or higher. Catalyst concentrations generally canrange from about 0.001 to about 1 weight percent chromium based on theweight of reactor contents. The catalyst is kept in suspension and iscontacted with the monomer(s) in an organic medium inert in the processat a pressure sufficient to maintain the medium and at least a portionof the monomer(s) in the liquid phase. The organic medium and thereaction temperature are selected such that the polymer is produced assolid, discrete particles and is recovered in that form. The organicmedium employed is generally a paraffin and/or cycloparaffin having from3 to 12 carbon atoms per molecule. Representative examples includespropane, n-butane, isobutane, pentane, isopentane, n-dodecane,cyclohexane, methylcyclohexane, and the like.

If desired, hydrogen can be employed in the polymerization process toregulate or decrease the molecular weight of the polymers produced withthe invention catalysts, e.g., give higher melt index products asdetermined in accordance with the procedure of ASTM D 1238-65T,condition E. When hydrogen is used its partial pressure can range fromabout 20 to about 120 psi (0.14-0.83 MPa).

High pore volume of the catalyst support is generally considereddesirable in ethylene polymerization catalysts based on silica-supportedchromium oxide since the ability of the catalysts to produce polymerswith high melt index, particularly in a particle form polymerizatinprocess, in partly dependent upon attaining high pore volumes. As ageneral proposition, the melt index of ethylene polymers produced withthese catalysts is also related to the polymerization temperature, thehigher the temperature the higher the melt index. In a particle formprocess, however, a relatively low maximum reaction temperature is used,otherwise polymer can go into solution and its recovery becomes timeconsuming and expensive. The relatively between catalyst pore volume andpolymer melt index is detailed in U.S. Pat. No. 4,169,926.

The following examples are intended to further illustrate the inventionand show some preferred details but are not intended to unduly limit thescope thereof.

EXAMPLE I

A sample of commercially available, purified silica hydrogel containingabout 28 weight percent solids was diluted with sufficient water toproduce a hydrogel slurry masterbatch containing about 20 weight percentsolids. The pH of the masterbatch was essentially neutral, e.g., 6 to 8.In general, an 80 ml (104 g) portion of the hydrogel contained in anopen mouth jar was admixed with the specified acid or base, if used.Each jar was then placed in a vacuum oven set at 150° C. and a pressureof about 10 to 15 inches Hg (34-51 kPa) where it remained overnight(about 15 hours) for slow drying. The dried samples were removed,individually ground until each passed through a 50 mesh screen (U.S.Sieve Series) and the ground material calcined in air at 500° C. forabout 16 hours. Pore volumes were determined from samples of therecovered, calcined products according to the Innes method usingisopropanol.

In one series of runs the effect of pH level of the silica hydrogel onpore volume of the subsequently dried and calcined gels obtained fromeach sample was determined.

In a second series of runs the effect of sulfuric acid concentration ofthe silica hydrogel on pore volumes of subsequently dried and calcinedgels made from each sample was determined.

In a third series of runs the effect of various acids individually usedin the silica hydrogel on pore volumes of subsequently dried andcalcined gels produced from each sample was determined.

In a fourth series of runs the effect of various organic acidsindividually used in the silica hydrogel on pore volumes of subsequentlyslowly dried and calcined gels prepared from each sample was determined.The acids dissolved unless noted to the contrary. The specificadditive(s) used in each run and concentration thereof and pore volumeresults are given in Tables 1A, 1B, 1C and 1D.

                                      TABLE 1A                                    __________________________________________________________________________    Effect Of pH On Silica Gel Pore Volume                                           Hydrogel                                                                      Add.              Hydrogel                                                                              Pore                                             Run                                                                              Vol.              Add.    Vol.                                                                             Improv..sup.(a)                               No.                                                                              Ratio                                                                              Hydrogel Additive                                                                          Wt..sup.(f)                                                                        pH g/ml                                                                             %     Remarks                                 __________________________________________________________________________    1  --    none        --   6.8                                                                              1.33                                                                             --.sup.(b)                                                                          control                                 2  --   "            --   "  1.32                                                                             --    "                                       3  --   "            --   "  1.32                                                                             --    "                                       4  --   "            --   "  1.34                                                                             --    "                                       5   80  Conc. NH.sub.4 OH.sup.(c), 0.90g                                                           116  9  1.34                                                                             --    "                                       6  210  NMe.sub.4 OH,.sup.(d) 0.08g                                                                1300 9-10                                                                             1.32                                                                             --    "                                                                 (est.)                                              7   80  NMe.sub.4 OH,.sup.(d) 0.2g                                                                 520  9-10                                                                             1.29                                                                             -3    "                                                                 (est.)                                              8  210  Conc. H.sub.2 SO.sub.4.sup.(e), 0.67g                                                      155  2.2                                                                              1.50                                                                             13    invention                               9  160  Conc. H.sub.2 SO.sub.4.sup.(e), 0.92g                                                      113  1.8                                                                              1.88                                                                             41    "                                       __________________________________________________________________________     NOTES:                                                                        .sup.(a) Relative to baseline of 1.33 g/cc.                                   .sup.(b) A dash indicates no entry is needed.                                 .sup.(c) 28% ammonium hydroxide.                                              .sup.(d) Tetramethylammonium hydroxide, 20 wt. % in water.                    .sup.(e) 96% sulfuric acid.                                                   .sup.(f) The additive weights shown are calculated on the basis of 100%       active component.                                                        

The results presented in Table 1A in control runs 1-4 show that silicahydrogels at an initially neutral pH when oven dried yield silica gelshaving a relatively low pore volume of about 1.3 g/cc. Control runs 5-7,show similar results for silica gels produced from the hydrogels at amoderately basic, initial pH level of about 9 to about 10. However,invention runs 8-9 show that silica hydrogels slowly dried at amoderately acidic, initial pH value of about 2 yield silica gels havingsubstantially improved pore volumes. Invention run 8 shows a pore volumeof 1.50 g/cc for the silica gel relative to the control gel pore volumeof 1.33 g/cc (13% improvement) and the gel of invention run 9 shows apore volume of 1.88 g/cc (41% improvement) relative to the control gel.

The data in Table 1A demonstrate that silica gels having increased porevolumes are produced from the corresponding silica hydrogels containingabout 20 weight percent solids, said hydrogels exhibiting a moderatelyacidic pH of about 2 prior to drying them conventionally in an air oven.The presence of the acid reduces shrinkage of the hydrogel as it driesin the presence of water and acid.

It is disclosed in U.S. Pat. No. 4,169,926 that ethylene polymerizationcatalysts consisting of chromium oxide supported on silica yieldpolymers, in a particle form polymerization process, which showincreasing melt index values as the pore volume of the supportincreases. Thus, ethylene polymerization catalysts comprising chromiumoxide supported on the improved silica gels of the instant inventionwill exhibit greater melt index capability than catalysts derived fromthe control silica gels.

                  TABLE 1B                                                        ______________________________________                                        Effect Of H.sub.2 SO.sub.4 Concentration On Silica Gel Pore Volume                              Hydrogel                                                                     Additive     Pore                                            Run  Con. H.sub.2 SO.sub.4                                                                     Wt.          Vol. Improv.                                    No.  Wt. gms.sup.(a)                                                                           Ratio    pH  g/ml %.sup.(b)                                                                            Remarks                             ______________________________________                                         1   none        --       6.8 1.33 --.sup.(c)                                                                           control                             10    0.40       260      1.4 1.45  9     invention                            8    0.67       155      2.2 1.50 13     "                                    9    0.92       113      1.8 1.88 41     "                                   11   1.8         58       1.5 1.79 35     "                                   12   1.8         58       1.5 1.77 33     "                                        (repeat)                                                                 13   3.7         28       1.2 1.74 31     "                                   14   5.5         19       1.0 1.72 29     "                                   15   9.2         11       0.8 1.66 25     "                                   16   18.4         6       0.3 1.63 23     "                                   17   1.8 (neutralize                                                                           58       7.  1.39  5     control                                  with NH.sub.4 OH)                                                        ______________________________________                                         .sup.(a) Calculated amount of 100% H.sub.2 SO.sub.4 in the 96% acid used.     .sup.(b) Relative to baseline of 1.33 g/cc.                                   .sup.(c) A dash signifies no entry is needed.                            

                                      TABLE 1C                                    __________________________________________________________________________    Effect Of Various Acids On Silica Gel Pore Volume                                              Hydrogel                                                                     Additive Volume                                               Run             Wt.      Pore Improv.                                         No.                                                                              Acid Employed                                                                              Ratio                                                                              pH  g/ml %.sup.(a)                                                                          Remarks                                    __________________________________________________________________________     1   none       --   6.8 1.33 --.sup.(f)                                                                         control                                    18 Con. HCl.sup.(b), 0.38g                                                                    274  1.1 1.45 9    invention                                  19 Con. HCl.sup.(b), 0.43g                                                                    242  1.0 1.83 38   "                                          20 Con. HCl.sup.(b), 0.86g                                                                    121  1.0 1.83 38   "                                          21 Con. H.sub.3 PO.sub.4.sup.(c), 1.4g                                                         74  1.7 1.70 28   "                                          22 Sulfamic, 1.0g                                                                             104  1.5 1.80 35   "                                                               (calc)                                                   23 Con. HNO.sub.3.sup.(d), 0.9g                                                               116  1.3 1.88 41   "                                          24 Glacial acetic acid, 2.1g                                                                   50  3.3 1.62 22   "                                                               (calc)                                                                            (1.70).sup.(e)                                                                     28                                              __________________________________________________________________________     NOTES:                                                                        .sup.(a) Relative to baseline of 1.33 g/cc.                                   .sup.(b) 36% HCl                                                              .sup.(c) 85% H.sub.3 PO.sub.4                                                 .sup.(d) 69.5% HNO.sub.3                                                      .sup.(e) Repeat determination.                                                .sup.(f) A dash signified no entry is needed.                            

                                      TABLE 1D                                    __________________________________________________________________________    Effect of Assorted Acids on Silica Gel Pore Volume                                                 Hydrogel                                                                     Additive                                                                           Pore                                                 Run                                                                              Acid Employed    Wt.  Vol.                                                                             Improv.                                           No.                                                                              Wt. gms.         Ratio                                                                              g/cc                                                                             %.sup.(a)                                                                          Remarks                                      __________________________________________________________________________     1 none             --   1.33                                                                             --.sup.(e)                                                                         control                                      25 citric, 2        52   1.70                                                                             28   invention                                    26 .sup.(b) CPTA, 2 52   1.72                                                                             29   "                                            27 ascorbic, 2      52   1.67                                                                             26   "                                            28 malic, 2         52   1.74                                                                             31   "                                            29 succinic, 2      52   1.79                                                                             35   "                                            30 gluconic, 2      52   1.61                                                                             21   "                                            31 diglycolic, 2    52   1.73                                                                             30   "                                            32 add NH.sub.4 OH to 31 to pH 7                                                                  --   1.30                                                                             -2   control                                      33 add NH.sub.4 OH to 32 to pH basic                                                              --   1.33                                                                              0   "                                            34 tartaric acid, 2 52   1.79                                                                             35   invention                                    35 like 34 + 0.67g conc. H.sub.2 SO.sub.4                                                         40   1.83                                                                             38   "                                            24 glacial acetic acid, 2.1                                                                       50   1.62                                                                             22   "                                            36 like 24 + 0.67g conc. H.sub.2 SO.sub.4                                                         38   1.92                                                                             44   "                                            37 .sup.(c) nitrilotriacetic, 2                                                                   52   1.75                                                                             32   "                                            38 .sup.(d) add NH.sub.4 OH to 37 to basic pH                                                     --   1.32                                                                              0   control                                      39 add H.sub.2 SO.sub.4 to 38 to pH of 2                                                          --   1.54                                                                             16   invention                                    __________________________________________________________________________     NOTES:                                                                        .sup.(a) Relative to baseline of 1.33 g/cc                                    .sup.(b) cyclopentane tetracarboxylic acid                                    .sup.(c) the acid, N(CH.sub.2 COOH).sub.3, apparently did not dissolve,       nevertheless it improved pore volume                                          .sup.(d) the nitrilotriacetic acid dissolved                                  .sup.(e) A dash signified no entry is needed.                            

The results presented in Table 1B in invention runs 8-16 show therelationship between hydrogel pH and pore volume of gels produced fromthe oven dried hydrogels. In these runs, the pH of the hydrogel samplesranged from 0.3 to 2.2. The gel with the highest pore volume had aninitial pH of 1.8. A pH levels of about 1.8 or greater for thehydrogels, the pore volumes of the oven dried gels rapidly declined. AtpH levels of from about 1.8 to about 0.3 for the hydrogels, the porevolumes of the oven dried gels slowly decrease from about 1.9 to about1.6.

In control run 17 of Table 1B, the results indicate that pore volumeincrease is related to the hydrogel pH level and not to the presence ofthe sulfate anion. The hydrogel is first acidified with concentrated H₂SO₄ to a pH value of about 1.5 as shown in runs 11, 12 and then themixture is neutralized with ammonia so that the sulfate anion remainedin solution. Silica gel produced from the oven dried hydrogel is shownto have a pore volume increase of only about 5%. The low pore volume ofthe gel indicates that the pore volume increase is pH related and isreversible.

The results given in Table 1C demonstrate the effect of various acids onthe pore volume increase of silica gels oven dried from thecorresponding hydrogels. The results show that strong acids such as HC1,HNO₃, H₃ PO₄ and sulfamic acid all greatly increase the pore volume ofthe gels oven dried from the acidified hydrogels. In run 24, a weak acidsuch as acetic acid also is effective.

The results given in Table 1D demonstrate that a variety of organicacids are effective in silica hydrogels. Such mixtures when oven driedyield gels which exhibit substantially larger pore volumes thanuntreated hydrogels. Controls runs 32, 33 (compare with invention run31) and control run 38 (compare with invention runs 37, 39) againdemonstrate the effect of pH and show that it is reversible.

All of the acids employed are water soluble except nitrilotriacetic (run37) which apparently did not dissolve. Yet, the gel produced in run 37has an increased pore volume 32% better than the control. The reason forthe behavior of the acid is not understood at this time.

The hydrogel: additive(s) weight ratio can vary widely depending uponthe nature of the additive. With strong acids the weight ratio can rangefrom about 5 l to about 275. With weak acids it can range from about 5to about 60 or higher.

EXAMPLE 2

Individual 100 gram portions of a silica-titania cogel hydrogelcontaining 14 weight percent solids and 2 weight percent titanium astitania based on the dry cogel were pushed through a 15 mesh screen andeach sample slurried with 100 ml of water. The cogel hydrogel wasprepared as disclosed in U.S. Pat. No. 3,870,656. Each sample, aftertreatment, if used, was dried, calcined, and pore volume measured asdescribed in Example 1.

The results are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Effect Of Acid On Pore Volume Of Silica-Titania Cogel                                       Hydro-                                                                        gel     Ap-                                                     Acid Employed Acid    prox.   Pore Im-                                        Run  For-               Wt.   pH    Vol. prov. Re-                            No.  mula     g     ml  Ratio Calc. g/cc %     marks                          ______________________________________                                        40   H.sub.2 SO.sub.4                                                                      0.92   0.5 435   1.8   0.95 9     in-                                                                           vention                        41   --.sup.(a)                                                                            0      0   --    7     0.87 --    control                        ______________________________________                                         .sup.(a) A dash signifies no entry is needed.                            

The data presented in Table 2 indicate that some improvement is obtainedby treating silica cogel hydrogels with acids. The difference inbehavior between silica cogel hydrogels and silica hydrogels as far asacid treating goes is believed to be inherent to the gel. That is,silica hydrogels can be prepared containing up to about 30 weightpercent solids whereas silica cogel hydrogels can contain up to about 14weight percent solids. Thus, the cogel hydrogel structure (primaryparticles) possesses larger pores, less mechanical resistance to shrinkduring drying, and possibly the repulsion of the primary particles dueto charge buildup during acidifying is lessened relative to silicahydrogel.

Reasonable variations and modifications which will become apparent tothose skilled in the art can be made in this invention without departingfrom the spirit and scope thereof.

We claim:
 1. A process to produce an olefin polymerization catalystcomprising:(a) contacting an aqueous alkali metal silicate comprisingsolution with an acid to produce an alkali-containing hydrogel, (b)removing alkali from said alkali-containing hydrogel by washing withwater to obtain a purified silica-containing hydrogel, (c) admixing theso obtained purified silica-containing hydrogel with acid to obtain anacidified silica-containing hydrogel having a pH of 3.5 or less, (d)maintaining said acidified silica-containing hydrogel at a temperaturenear the boiling point of water at the drying pressure until essentiallyall water is evaporated (e) prior to, during or after said dryingutilizing a chromium source to obtain a chromium-containing gel, (f)recovering a chromium-containing xerogel as the product of the process.2. A process in accordance with claim 1 comprising:(a) reacting alkalimetal silicates and a strong acid to produce an alkali ions-containinghydrogel, (b) washing said alkali ions-containing hydrogel to removeessentially all of the alkali ions to produce a purified hydrogel, (c)mixing said purified hydrogel and an acid to produce an acidic hydrogel,(d) drying said acidic hydrogel.
 3. A process in accordance with claim 2comprising aging said purified hydrogel prior to the mixing of saidpurified hydrogel with said acid.
 4. A process in accordance with claim1 comprising slowly drying said silica-containing hydrogel.
 5. A processin accordance with claim 4 comprising dissolving said chromium compoundin said acid and mixing the solution so obtained and said purifiedhydrogel.
 6. A process in accordance with claim 1 comprising mixing saidlarge pore volume xerogel and a solution of a chromium compound in aninert organic solvent and removing said solvent to obtain saidchromium-containing xerogel.
 7. A process in accordance with claim 1wherein said silica-containing hydrogel consists essentially of silica.8. A supported chromium polymerization catalyst produced by the processof claim 1 the support of which comprises silica xerogel having a porevolume of above about 1.4 cc/g.
 9. A process in accordance with claim 1wherein said step (b) is carried out under a radiant heat source or in adrying vacuum oven.
 10. Process of claim 1 wherein said acid used instep (c) is an organic acid.